Lithium secondary batteries, which are recently in the spotlight as a power source of portable and small electronic devices, are batteries exhibiting high energy density, in which high discharge voltages that are two times or more than those of typical batteries using an alkaline aqueous solution may be exhibited by using an organic electrolyte solution.
Graphite is mainly used as an anode material of a lithium secondary battery. However, graphite has a low capacity per unit mass of 372 mAh/g and a high-capacity lithium secondary battery may be difficult to be prepared by using graphite.
As an anode material exhibiting higher capacity than graphite, a material forming an intermetallic compound with lithium, such as silicon, tin, and oxides thereof, may be promising. However, volumes of the above materials may expand because crystal structures thereof may be changed when absorbing and storing lithium. When silicon absorbs and stores the maximum amount of lithium, the silicon may be transformed into Li4.4Si and the volume of Li4.4Si may expand due to charging. With respect to the rate of increase in volume due to the charging, the volume may expand up to about 4.12 times the volume of the silicon before the volume expansion.
Also, research into achieving a buffering effect on the volume change by using nanoscale silicon particles or using porous silicon has been conducted. With respect to the nanoparticles, a method of coating metal nanoparticles with carbon has been studied.
However, manufacturing costs of nanoparticles may be high, cracks may occur in carbon simultaneously with the expansion of metal during charging due to the brittleness of carbon, and a space may be generated between the carbon and the metal in the process of being shrunk again during discharging. Thus, an effect of improving lifetime may be insignificant.
Also, research into decreasing a volume expansion coefficient by alloying silicon has been conducted as research for increasing the capacity of an anode material such as silicon. However, since a metal, such as silicon (Si), tin (Sn), and aluminum (Al), may be alloyed with lithium during charge and discharge, volume expansion and contraction may occur. Thus, cycle characteristics of the battery may degrade.